Mechanical alloying is a process of repeated fracturing and welding used to form alloys of unique composition, morphology and structure. Mechanical alloying is capable of producing dispersion strengthened alloys that are not producible by casting, rapid solidification or even conventional powder metallurgy techniques. Mechanical alloying has been commercially used to produce dispersion strengthened aluminum, iron and nickel-base alloys. Commercially available dispersion strengthened alloys having significantly improved properties arising from mechanical alloying include alloys such as MA 754, MA 956, MA 6000 and AL-905XL.
During mechanical alloying it is essential to control the welding and fracturing of powders. If powder welds excessively, powder will agglomerate in a mill to form an unworkable heap of powder prior to mechanical alloying. If powder fractures excessively, ultra fine unalloyed particles are formed. Under extreme excess fracture conditions, ultra fine metal powders may become pyrophoric. A process control agent (PCA) is used to balance welding and mechanical fracturing to achieve the desired mechanical alloying. The PCA additives used may be any organic material such as organic acids, alcohols, heptanes, aldehydes and ether. Process control agents may also be a material such as graphite, oxygen and water. Typically, fugitive PCA's partially combine with metal powder during mechanical alloying to form dispersoid strengtheners. Excess PCA (fugitive PCA) must be removed prior to consolidation of canned mechanically alloyed powder. Excess PCA is commonly removed by argon purging followed by a vacuum degas treatment at elevated temperature. After degassing, a consolidation technique such as hot extrusion or hot isostatic pressing is typically used to form degassed mechanically alloyed powder into a metal product.
A conventional PCA such as stearic acid [CH.sub.3 (CH.sub.2).sub.16 COOH], is not useful for mechanical alloying titanium. During mechanical alloying, stearic acid breaks down to introduce oxygen into the milling atmosphere. Oxygen is readily dissolved into the titanium matrix. Dissolved oxygen in titanium rapidly deteriorates mechanical properties. A graphite process control agent also is not always useful for controlling mechanical alloying of titanium-base alloys. Elemental carbon has a very low solubility in titanium. Furthermore, carbon reacts with titanium for form TiC only at a relatively high temperature of above about 1000.degree. C.
Alternatively, temperature may be used to control mechanical alloying. Milling temperature is a factor which controls welding rate during mechanical alloying. Typically, welding rate increases with increased temperature. For example, a liquid nitrogen cooling jacket surrounding a mechanical alloying device has been used to decrease operating temperature for suppressing welding of metal powders. The problem with using a cooling jacket for controlling mechanical alloying is that it is difficult to effectively lower temperature within large vessels that are required for commercially viable operations. In addition, others have added liquid nitrogen directly into a mill for mechanical alloying. The problem with controlling a mechanical alloying operation with liquid nitrogen in the mill is that nitrogen combines with metal powders to adversely affect properties. Nitrogen typically is a harmful ingredient to most alloy systems including titanium-base alloys.
It is an object of this invention to provide an improved process control agent for mechanical alloying titanium-base metal powders.
It is a further object of this invention to provide a method of controlling mechanical alloying without introducing excess oxygen, carbon or nitrogen into a titanium-base matrix.
It is a further object of this invention to provide a process control agent that improves physical properties of titanium-base alloys.